The present application relates to backlighting in liquid crystal displays (LCDs). More specifically, it relates to techniques of synchronizing the operation of multiple, independent, light-producing elements to enhance the apparent quality of moving images displayed on the LCD video display and will be described with particular reference thereto. It is to be appreciated that the present application is also applicable to other systems that utilize backlights, and is not limited to the above-referenced application.
Generally, in an LCD monitor, pixel intensity is controlled by controlling the amount of light that is let through the surface of the display. The liquid crystal elements are controlled by applying current to them, thereby creating dark pixels, or light pixels, or intermediate shades. The liquid crystal elements do not typically produce any light of their own, rather the visible portion comes from an array of backlights, and the liquid crystal elements selectively let that backlighting show, producing a visible image. Typically, these backlights have been cold cathode fluorescent lamps.
A moving image is produced on an LCD video display by sequentially updating the picture elements (pixels) at a rate that is somewhat faster than human perception. This rate, referred to as the scan rate of the video, is generally either 50 Hz or 60 Hz, depending on geographical region. It is generally known that the apparent sharpness of the moving image can be significantly improved by illuminating the pixels with the backlight only when the pixels have assumed a stable, unchanging state. As a consequence, the backlighting to the pixel must be extinguished during the finite time required to update the pixel to produce the next subsequent image in the video frame.
This technique had been demonstrated in commercially available LCD video displays using fluorescent tubes to backlight the LCD screen. Each lamp is systematically extinguished while the rows of pixels that it illuminates are updated. When the pixels in those rows have transitioned to form a stable image, the fluorescent tube is re-illuminated to reveal the LCD image to the observer. Each fluorescent lamp performs this action while each horizontal band across the video monitor is refreshed to display the next frame in the video. Since this action occurs according to the scan rate, the extinguishing and subsequent re-illumination of the fluorescent lamp is beyond the limits of human perception, producing a moving video image with apparently constant light intensity that is proportional to the time interval over which each fluorescent tube is illuminated. It can be appreciated that the average brightness of the observed image can be modulated up or down by modulating the on-off duty cycle of the fluorescent lamp.
To date, scanning has been accomplished in LCDs. Current systems handle synchronization and dimming control on the scanning backlight with a single large pin-out microcontroller, as shown in FIG. 1. This one microcontroller contains the scanning and dimming algorithm for all of the backlights, of which 12 lamps is a typical number. Typically, there is one inverter ballast for every lamp, that is, every lamp is being driven by its own power electronics circuit. To dim the lamp, a pulse width modulated (PWM) signal is used. When the PWM is off, it turns the lamp off. When the PWM is on, it turns the lamp on. To control dimming, a PWM signal of a length corresponding to dimming (i.e. the desired brightness of the lamp) is fed to each inverter corresponding to each lamp. Each lamp is offset by a certain amount, so that when the display scans down, it follows the visible pattern of the video image panning over the screen.
Several problems arise when using a single microprocessor to control the scanning of several backlights. First, at least one pin for each inverter (lamp) is required. The software involved to control such a system is relatively complex, and typically the actual processor is larger with more memory. The actual physical profile of the processor is also quite large, typically having a 64 pin configuration. Another drawback is that a single processor of this size is completely dedicated to the scanning control. It typically does not house enough processing capability to perform additional functions, such as end of life calculations, preheating and dimming of the lamps, and other lamp maintenance functions that are desirable in general, but not necessarily related to scanning.
Scanning PWM pulses can improve motion blur on LCD television screens. The main problem is how to handle the scanning requirement in a cost effective, power efficient and space efficient manner. An algorithm to run scannable dimming on twelve lamps is complex and computationally intensive. On top of this, there should be other functionality embodied in these processors to save cost and components.
Another problem is power consumption. Generally, the more tasks a single processor performs, the more power it draws, but inordinately more than the added functionality provided. Another problem lies in arrangement of the circuit. Physical layout of a circuit implementing a single processor controlled scanning system can be quite complex and cumbersome. Moreover, potential for failure is increased in a single processor system.
As the number of independent light producing elements increases, the computational intensity of synchronizing the light sources also increases. Since the scan rate is fixed, the amount of time during which calculations must be performed is likewise fixed. This places a significant demand on the capability of the microcontroller, particularly in large applications that require the use of many lamps. As the size of the display is scaled up, the capability and expense of the microcontroller increases. The display could even be scaled up to a point where the calculations required are beyond the capability of commercially available microcontrollers.